We report the results of ASCA and Ryle Telescope observations of the distant cluster of galaxies A1204. The X-ray image shows that this source is very compact and that the intracluster gas is strongly centrally concentrated. The brightness temperature decrement of the cosmic microwave background radiation toward this cluster (the Sunyaev-Zeldovich effect) was not detected by the Ryle Telescope, with a 1 σ upper limit of |ΔT| < 360 μK. The gas temperature is 3.6 keV, which is considerably lower than that expected from the bright X-ray luminosity in the ROSAT band and explains the lack of detection. The hot gas mass within 1 Mpc is estimated to be ≲7 × 1013 M0. In the wide window of the ASCA Gas Imaging Spectrometer, a very faint X-ray source was found at R.A. = 11h12m16s, decl. = +17°56′40″ (J2000). The observed count rate was too small to identify the X-ray emission mechanism in the new source.
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